The invention relates to an electron tube. The tube has a tubular envelope portion which at one end has a bearing surface normal to the axis of the tube. A window having a radiation-sensitive layer thereon bears on the bearing surface. There is no sealing means at the scan between the window and the bearing surface. Instead, the seam is sealed hermetically by a mass of indium or an indium alloy.
The invention further relates to a method of manufacturing this electron tube.
Such an electron tube is disclosed in U.S. Pat. No. 3,243,627. The envelope is connected to the window and the seam is sealed by molding a ring of indium or a eutectic alloy of tin and indium in a mold placed around the envelope and window. This process is carried out in a vacuum bell jar which is placed in an oven. All tube parts are subjected to a temperature which is at least equal to the melting temperature of the ring, in this case indium or a tin-indium alloy.
This means that the radiation-sensitive layer provided on the window is also heated to a temperature higher than 100.degree. C. Radiation-sensitive layers, and in particular photosensitive layers of television camera tubes, however, generally cannot readily withstand such temperatures.
U.S. Pat. No. 3,543,383 discloses a method in which the indium or alloy ring is melted by inductive heating. The ring is present between the window and the bearing surface of the tubular envelope portion. During the inductive heating of the indium ring, the envelope and window are vibrated ultrasonically so as to break the oxide skin present on the indium.
The provision of the indium ring between the window and the bearing surface of the envelope is a problem for those tubes in which the window and the radiation-sensitive layer provided thereon must be positioned very accurately relative to other electrodes in the tube. An example of such a tube is one having a gauze electrode at a short distance from the photosensitive layer. Very narrow tolerances apply as regards plane-parallelism and spacing of the photosensitive layer with respect to the gauze electrode.
In the tube described in U.S. Pat. No. 3,543,383, the indium seal also determines the accuracy of the plane-parallelism and spacing. Moreover, sealing material may flow inward between the window and the bearing surface during the sealing process. As a result of this, undesired electron-optical disturbances may be produced in the operating tube, for example disturbances in the pattern of the electrical field lines.